The invention relates to an electroacoustic communications unit such as a mobile telephone and a telephone handset to be held in engagement with a user""s ear, but also a headphone.
Particularly in the field of mobile telephones there has been a development toward units which both weigh less and less and are less and less bulky. This makes great demands on designers and manufacturers of electronic and electroacoustic components, which here comprise microphone and sound generator or receiver transducers. These transducers, too, are available toady in smaller dimensions than before.
The electroacoustic receiver transducers used here have a diaphragm for generating acoustic signals in the form of sound with frequencies in the audible range. The transducer is typically arranged closely adjacent to the inner side of the housing of the telephone, there being acoustic connections in the form of apertures whose shape is carefully adjusted to give the correct acoustic impedance and frequency characteristic in engagement with the user""s ear. The diaphragm moves and emits sound from its front side, and for the transducer to operate satisfactorily, it is necessary that an air volume of a certain size is available to the rear side of the diaphragm, since the diaphragm will otherwise be blocked and prevented from moving. A too small air volume available to the rear side of the diaphragm will reduce the sensitivity of the transducer and will result in a frequency characteristic having an unfortunate course in the form of a notch. Therefore, so far, mobile telephones and telephone handsets have necessarily had such a volume in the housing, and the size of this volume has so far been a limitation for their designers and manufacturers.
Mobile telephones and telephone handsets are tested for their acoustic quality using an ear simulator which is standardized, e.g. according to IEC 318, IEC 711 or corresponding standards, all of which are based on the situation of use, where a user holds the telephone or the handset against his ear. To ensure reproducible measurements, these standards provide guidelines for the mechanical structure of the ear simulator and its acoustic function with a view to simulating a human ear as best as possible, and it is laid down how to carry out a test, including how to engage the telephone with the ear simulator, and in particular the accurate position and orientation of the telephone relative to the ear simulator. This means that, for a given telephone, and on the basis of the standards, it is possible to define the engagement face of the telephone with the ear simulator in the standardized test, which, according to the standard, will correspond to engagement with a user""s ear. In the following, the expression xe2x80x9cface for engagement with a user""s earxe2x80x9d will therefore be used as a synonym for the engagement of the communications unit with a standardized ear simulator, as these faces are identical according to the foregoing. Such standardized ear simulators are commercially available e.g. from Brxc3xcel and Kjxc3xa6r as type 4157, type 4185 and type 4195. The engagement face is typically a circular area with a diameter of 25 mm, the ear simulator having a ring-shaped engagement face with a 25 mm diameter. Inwardly of the ring-shaped engagement face of the ear simulator there is an air volume corresponding to the volume of the human outer ear. This volume is the sound entrance opening of the ear simulator to a microphone by means of which the sound from the telephone may be registered.
The size of the air volume in the sound entrance of the ear simulator essentially influences the acoustic loading of the receiver transducer of the communications unit. Any leakage to the surroundings may cause a considerable increase in this volume, which gives a significant change in the acoustic loading. Changes in the acoustic loading may cause measurement results which do not correspond correctly to the acoustic properties of the communications unit which it is desired to measure. To achieve correct and reproducible measurements which are characteristic of the communications unit, the communications unit must therefore, in accordance with the standards, be in a tight fitting relationship with the ring-shaped engagement face of the ear simulator.
In use, there will be considerable variations in the degree of the tightness of the engagement of the communications unit with the user""s ear, which gives greater or smaller acoustic leakages to the surroundings. Such variations influence the acoustic loading of the receiver transducer, which in turn, depending on the unit""s sensitivity to the acoustic loading impedance, may change the acoustic properties of the communications unit. Such changes are perceived by the user as variations in the sound quality of the communications unit. This is undesirable.
Accordingly, there is a need for communications units which are leakage-tolerant, that is units which give a perceived sound quality for the user as well as measurement results on an ear simulator all of which, to the greatest extent possible, are independent of leaks or leakages both in the engagement of the unit with the user""s ear and with the ear simulator.
Mobile telephones are made ever smaller, and telephones having a very small inner air volume cannot, with the prior art, readily be made leakage-tolerant, as leakage tolerance requires that the transducer has access to a certain air volume behind the transducerxe2x80x94typically some cm3. A very large inner air volume is undesirable, since this will increase the size of the unit, and, as an alternative to a large inner air volume, acoustic openings could be established from a small inner air volume to the ambient air outside the telephone. The transducer would hereby have access via the small inner air volume to an infinitely large air volume, and a leakage-tolerant function could be achieved. Such a structure with a small inner volume will form an acoustic resonance circuit consisting of the acoustic inductance of these acoustic openings and the acoustic capacitance of the small inner air volume. Such a resonance circuit will cause the frequency response of the telephone to have a relatively deep and-sharp notch, which will unfortunately be in the middle of the useful audio frequency range and will be determined by the geometry of the openings and the inner air volume.
EP 364 935 describes a telephone handset with a receiver transducer, where the front side of the diaphragm is acoustically connected with the user""s ear through acoustic openings in the handset, and moreover the front side of the diaphragm is acoustically connected with the inner cavity in the handset.
DE 2 815 051 and U.S. Pat. No. 4,239,945 describe various headphones, where an acoustic connection is see up between the front side of the diaphragm and its rear side, said connection including the volume in the user""s outer ear.
WO 98/24214 describes a mobile telephone, where, inwardly of the telephone face for engagement with a user""s ear, there is an acoustic connection to the front side of the diaphragm and an acoustic connection to the rear side of the diaphragm, whereby the telephone becomes leakage-tolerant. The receiver transducer is arranged eccentrically relative to the face for engagement with the user""s ear.
All the above-mentioned known structures have a considerable air volume in the housing behind the diaphragm.
With a communications unit according to the invention, four acoustic connections are established from the diaphragm to the outer side of the housing, there being two connections from the front side of the diaphragm and two connections from its rear side, and both the front side and the rear side of the diaphragm are connected via respective acoustic connections with both the area inwardly of the face for engagement with a user""s ear, that is with the user""s ear, and with the area outwardly of this engagement face, that is with the ambient air. These four acoustic connections constitute the four branches in an acoustic balance bridge with the transducer as a generator.
By introducing a fourth acoustic connection between the front side of the diaphragm of the transducer and the air outside the communications unit, it is ensured that a leakage-tolerant unit may be constructed with a small inner air volume, as the effect of the acoustic resonance circuit may be cancelled to a great extent. The principle is that a leakage in the engagement with the user""s ear will contribute to a notch in the frequency response via the connection to the front side of the diaphragm, while the effect will be, via the connection to the rear side of the diaphragm, a corresponding peak in the frequency response at the same frequency. This compensation will greatly be independent of the size of the leakage.
Thus, an acoustic balance bridge has been established. The notch in the frequency response curve will vary in frequency from one telephone to another because of tolerances in the connections to the air outside the telephone, but the effect of the acoustic balance bridge is that leakage variations will give changes in two of the branches of the bridge, and it is here the same acoustic resonance circuit which is included in both of the branches of the bridge which are affected by leakages, whereby variations in both frequency and amplitude are balanced, and the bridge will still be in balance.
This configuration, where both the front side of the diaphragm and its rear side are acoustically connected both with the user""s ear and with the surrounding air, is particularly expedient, as it is hereby possible to reduce the inner air volume in the telephone behind the diaphragm of the transducer to a size which has no importance for the size of the telephone in practice. The small volume inevitably causes a relatively sharp resonance, that is with a relatively great amplitude variation in a relatively narrow frequency range, but, according to the foregoing, variations in both frequency and amplitude will be balanced.
In use, there will be variations in the degree of tightness of the engagement of the communications unit with the user""s ear. These variations occur for the individual user from time to time, and there may also be considerable variations from one user to the other. With the invention, these variations only influence the frequency characteristic to a very small extent, because both the front side of the diaphragm and its rear side are connected with this variable degree of tightness, which is thereby incorporated in two branches of the acoustic bridge.
Thus, with the invention it is possible to construct a communications unit which is leakage tolerant and which just has a quite small inner air volume.
Mobile telephones and telephone handsets frequently have a housing consisting of two shells which are assembled along their edges. This assembly will be more or less tight, and in any event it will be difficult and expensive to make it completely tight sealed. According to the invention, the acoustic connection between the rear side of the transducer diaphragm and the surrounding air may advantageously be provided in relation to this assembly, and it may even be formed as a discontinuity in the form of an incision in one of the edges of the shells at the assembly. Leakages, if any, in the assembly between the shells outside this discontinuity form part of this acoustic connection, and variations in the leakage are compensated to a great extent by the invention.